feat(clone): support credentialed git clone with askpass

Cloning now accepts optional username/password and passes them to the
Rust git layer via GIT_ASKPASS, avoiding interactive prompts. The UI
detects authentication failures, opens the credential dialog for clone,
and auto-hides error messages to keep the flow smooth.

- Add username/password support to clone_repository and git.ts
- Detect auth failures and route users to the clone credential dialog
- Improve clone UX with a dedicated loading overlay and timed errors

src/lib/git.ts

@@ -38,12 +38,16 @@ export function cloneRepository(
   remoteUrl: string,
   parentPath: string,
   directoryName?: string,
+  username?: string,
+  password?: string,
   commitLimit = 100,
 ): Promise<RepositoryBundle> {
   return invoke<RepositoryBundle>("clone_repository", {
     remoteUrl,
     parentPath,
     directoryName: directoryName?.trim() ? directoryName.trim() : null,
+    username: username ?? null,
+    password: password ?? null,
     commitLimit,
   });
 }